Periodicity

Question 1

What does the periodic table consist of?

Answer 1

All known elements, arranged in order of increasing atomic number
- Arrangement of elements reflects electronic structure of the elements
- Split into s,p,d,f blocks (elements in these blocks have their highest electron in the corresponding s,p,d,f electronic sub-level)

Question 2

Trend in Melting Point (Na–>Al) in period 3?

Answer 2

Increase as charge on ion and the number of delocalised electrons increases, so the metallic bonding gets stronger.

Question 3

Why does Si have the highest melting point in period 3?

Answer 3

As it has a macromolecular structure so has many strong covalent bonds to break.

Question 4

Why is there an overall decrease in melting point in period 3 (P–>Ar)?

Answer 4

Melting point is determined by the strength of the van der waals forces which vary with molecular size
S8>P4>Cl2>Ar

Question 5

Why is there an increase in boiling point across period 3 (Na–>Al)?

Answer 5

As charge on ion and number of delocalised electrons increases, so metallic bonding gets stronger.

Question 6

Why is Si b.p, not higher than Al?

Answer 6

As the covalent bonds have already been broken during melting

Question 7

Why does boiling point decrease in period 3 (P–> Ar)?

Answer 7

As Van der Waals forces are stronger as he molecular size inc increases:
S8>P4>Cl2>Ar

Question 8

What is the trend in atomic radius (nm)?

Answer 8

Decrease across the period as from Na-Ar nuclear charge increases but additional electrons go into same energy level so are attracted more strongly, atomic radius decreases.

Question 9

Trend in first ionisation energy across period 3?

Answer 9

General increase Na–>Ar, as nuclear charge increases, but shielding is constant.

Question 10

Anomalies in first ionisation energy trend across period 3

Answer 10

• Decrease Mg–>Al as outer electrons are more shielded than the 3p orbital
• Decrease P–>S as S has an electron paired up causing repulsion.